What medicine should you not mix with Benadryl?

Can you elaborate on the potential dangers of mixing Benadryl with other medications? Are there specific types of medicines that one should avoid combining with Benadryl, and why? Additionally, what are the potential side effects or reactions that may occur if Benadryl is mixed with these incompatible medications? Is it necessary to consult a healthcare professional before taking Benadryl if you are already on other medications?